Encased fluid filled transformer

ABSTRACT

Fluid filled electrical apparatus, such as transformers, and methods of constructing same, suitable for underground or vault mounting. The electrical apparatus includes a metallic tank or casing formed of mild or plain carbon steel, which has a sprayed metallic coating of corrosion-resistant material disposed on the external surfaces thereof.

United States Patent 1,769,562 7/ "Vi 99211:.- "ii-.1

Kuti et al. 51 Feb. 22, 1972 54] ENCASED FLUID FILLED 3,390,225 6/1968Couch'. ..l74/37 TRANSFORMER 3,405,283 10/1968 Leonard ..l74/37 X [72]Inventors: Albert J. Kuti, 1009 Woodview PL;

Thomas C. Junk, 3802 Morefield Road, primary 5 AGoldbel-gb0th0fShar0mPa-16146 Attamey-A. T. Stratton, F. E. Browder and Donald R.22 Filed: June 4,1970 i f y [21] Appl. No.: 43,532

- [57] ABSTRACT [52] U.S.Cl. ..174/l7LF, 174/37, 204/l96, Fluid filledelectrical apparatus, such as transformers, and 204/197, 336/94 methodsof constructing same, suitable for underground or 5 Int 5 00 vaultmounting. The electrical apparatus includes a metallic of n R 7. tank01' casing formed Of Ol' carbon steel, which has 4, 1, a sprayedmetallic coating of corrosion-resistant material disposed on theexternal surfaces thereof.

[35. .1 i&i 1@9, 7 Claims 3 Drawing FISIINS UNITED STATES PATENTSENCASED FLUID FILLED TRANSFORMER BACKGROUND OF THE INVENTION 1. Field ofthe Invention The invention relates in general to fluid filledelectrical inductive apparatus of the type which is disposed in highlycorros'ive environments, such as wholly or partially underground.

2. Description of the Prior Art Electrical apparatus disposed wholly orpartially underground, such as in specially designed vaults, or directlyburied in the earth, are subject to damage due to the corrosion of theirtanks or casings, which may seriously reduce the useful operating lifeof the apparatus. The corrosion mechanism of metals in water and soilhas been studied for many years, and different methods and structureshave been proposed for preventing or reducing the rate of corrosion.Electrical apparatus disposed below grade presents a much more difficultcorrosion protection problem than nonelectrical structures, such asunderground pipelines and buried tanks, because nonelectrical structuresmay be more easily protected from galvanic corrosion currents.Electrical components, on the other hand, are interconnected andgrounded, tending to increase the magnitude of corrosion currents, andthus the rate of corrosion.

The severity of the environment surrounding electrical apparatus mountedwholly or partially below grade, from the standpoint of corrosion,depends upon a large plurality of factors, such as the type of soil,moisture, heat, polluted water, acids, alkalies, minerals, bacteria, andthe like. Attempts to reduce corrosion in this environment have led toconstructing tanks for electrical apparatus of stainless steel, plastic,or other noncorrosive materials, and to the development of organicprotective coatings for tanks formed of mild steel.

Organic coatings over mild steel would be an economical answer to thecorrosion problem, if it could be assumed that the protective coatingwould have no pin-holes and would not be scratched during handling,shipment and installation. Unfortunately, it cannot be assumed thatthere will be no flaws or scratches in the protective coating, and sincemild steel disintegrates rapidly when subjected to a corrosiveenvironment, this combination is not a satisfactory solution to theunderground corrosion problem. In fact, the corrosive attack willusually be intensified at a flaw or scratch in the protective coating,which produces perforation of the tank faster than if the mild steel hadno protective coating.

Stainless steels are generally excellent from the galvanic corrosionstandpoint, but the stainless steels are not only costly, they increasefabrication costs, as they are more difficult to form, machine, and weldthan mild steel. Further, while the stainless steels are not subject toa general galvanic corrosion attack, they are susceptible to severelocalized attack under certain conditions, called pitting corrosion,which promotes stress-corrosion cracking.

Forming the casing of the apparatus from a material which includes mildsteel clad with a stainless steel outer layer, such as may be formed byheating billets of mild steel and stainless steel to the weldingtemperature and rolling them together, reduces the cost of the tankmaterial, compared to using all stainless steel. The stainless steellayer, however, is subject to severe localized attack, and the layer ofstainless steel may cause fabrication problems when the material isformed into the casing or tank of the electrical apparatus.

Thus, it would be desirable to provide new and improved electricalapparatus, such as transformers, which have corrosion resistant tankssuitable for underground mounting, but which may be manufactured withoutundue economic penalty due to greatly increased material cost and/orfabrication cost.

SUMMARY OF THE INVENTION Briefly, the present invention is new andimproved electrical apparatus, such as a transformer, which has a casingor tank suitable for mounting the apparatus in a highly corrosiveatmosphere, such as below grade in specially designed vaults, or

directly buried in the earth. The electrical apparatus has a metalliccasing, the external surfaces of which have a sprayed metal coatingdisposed thereon. The sprayed metal is a corrosion-resistant material,such as one of the stainless steels. A

first organic protective coating is disposed over the sprayed metalcoating, which impregnates and seals the pores of the sprayed metalcoating, which is inherently porous by nature. A second, more viscousorganic protective coating is applied over the first coating, to providea tough, moisture and chemical-resistant outer surface on the casing.The porous corrosion-resistant material, impregnated with resin,provides a cor rosion-resisting protective layer over the mild steeltank which is superior to a solid rolled cladding of the same material.The relatively porous structure of the sprayed metal, impregnated withan insulating resin, reduces galvanic corrosion currents, it eliminatesintergranular corrosion, it inhibits the formation of local galvaniccells which cause rapid pitting corrosion of stainless steel, and stresscorrosion cracking is also inhibited. Further, scratching of the outerprotective layer does not promote severed localized attack at theexposed area, as the metallic protective layer is a sprayed metalcoating impregnated with an organic material, which combinationsubstantially reduces corrosive attack.

New and improved methods of constructing the electrical apparatus arealso disclosed, which have the advantages of enabling the tank to becompletely fabricated before the sprayed metal coating is applied, whichmethods also protect welded joints, and the like. Further, the sprayedmetal coating, being inherently rough, promotes superior bonding of theorganic protective coatings thereto, compared with organic coatingsapplied to rolled-type cladded materials.

BRIEF DESCRIPTION OF THE DRAWING Further advantages and uses of theinvention will become more apparent when considered in view of thefollowing detailed description of exemplary embodiments thereof, takenin connection with the accompanying drawings, in which:

FIG. 1 is a perspective view, partially cut away, of an electricaltransformer disposed in an underground vault, which may be constructedaccording to the teachings of the invention;

FIG. 2 is a fragmentary, cross-sectional view of a tank or casing forelectrical apparatus, constructed according to the teachings of theinvention; and

FIG. 3 is a block diagram which illustrates the steps of a method ofconstructing electrical apparatus according to the teachings of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS The invention relates to new andimproved fluid filled electrical apparatus of the type mounted in ahighly corrosive environment, such as below grade level, and in generalis applicable to any electrical apparatus of this type having anelectrical element disposed in a tank or casing, with the electricalelement being adapted for connection to an external electricalpotential. Electrical transformers filled with mineral oil, askarel, SF6or the like, may utilize the teachings of the invention, as well asprotective apparatus, such as circuit breakers, which require acorrosion-resistant tank, and also capacitors which are intended foroperation in a corrosive environment.

FIG. 1 is a perspective view of an electrical transformer 10 of the typewhich may be constructed according to the teachings of the invention.Transformer 10 is a distribution transformer of the type commonly usedfor underground residential distribution, and it may be disposed in avault 12 below grade level 14, as illustrated, it may be disposed in avault which is partially below grade level, or it may be directly buriedin the earth, as required by the electrical utility.

Transformer 10 includes a casing 16 having a cover 18 which encloses thecore-winding assembly (not shown) of the transformer 10. A suitableinsulating and cooling fluid, such as mineral oil, is also disposed incasing 16, to insulate and vcool the electrical windings of thetransformer. Transformer [0 is hermetically sealed, with the electricalconnectors to the encased high voltage winding being made through thesealed high voltage bushing-connector assemblies 20 and 22, and throughthe sealed low voltage bushing assembly 24. The vault 12 has a heavyaccess cover 26, disposed thereon, which may be perforated to allowventilation.

Transformer is subjected to the highly corrosive atmosphere associatedwith below grade mounting, and must also withstand flooding of the vaultfor extended periods of time. Excellent organic protective coatings forunderground transformers have been developed, but since coatings are notalways pin-hole free, and since coatings may be scratched during thehandling and installation of electrical apparatus, coatings alone do notprovide the desired answer. Transformers and other fluid filledelectrical apparatus only require one hole through the casing or tank tocause costly damage and even failure of the apparatus. The fact thatmost of the casing may be corrosion free is of no benefit if one smallportion of the casing is severely attacked by corrosion. in fact, anexcellent protective coating with one scratch may cause failure of thetank at the exposed area faster than if the tank had no protectivecoating.

Constructing the. tanks of corrosion-resistant materials, such asstainless steel, is not desirable because of the economic penaltyincurred in the initial costof the material, and increased fabricatingcosts, and also because stainless steel, while not generally susceptibleto corrosion, may in certain environments be subject to a localizedpitting corrosion which is very rapid, perforating the casing at thepoint of attack while the majority of the surface is corrosion free.This localized attack may also promote stress-corrosion cracking ofstainless steel, which may cause failure of the tank even beforeperforation due to pitting occurs.

The present invention discloses new and improved fluid filled electricalapparatus, and methods of constructing same, which has a tank structurehighly resistant to corrosion, and it may be constructed without undueeconomic penalty. FIG. 2 is a fragmentary, cross-sectional view ofelectrical apparatus having a tank or casing 30 constructed according tothe teachings of the invention. The wall of tank 30 is shown greatlymagnified in order to more clearly illustrate the invention. Casing 30'may be the tank 16 or cover 18 of transformer 10 shown in FIG. 1, or thecasing of any fluid filled electrical apparatus having an electricallyconductive member therein adapted for connection to an external sourceof electrical potential.

Casing 30 includes a main or structural metallic layer 32, which formsthe base material of the tank, and this layer is disposed adjacent theinside of the casing, such as adjacent the fluid 34, a second,relatively thin metallic layer 36 of corrosion-resistant materialdisposed over the outer surfaces of the structural layer 32, and aprotective layer or coating 38 formed of an organic resin, disposed overthe second metallic layer 36. The protective layer 38, although beingshown as a single coating, is actually applied in two steps, as will behereinafter explained.

The base or inner layer 32 provides the complete structural requirementsof the casing 30. Since the mild or carbon steel commonly. used fortanks of electrical apparatus mounted above grade, such as S.A.E. 1010,is economically attractive, easy to fabricate, and possesses therequisite mechanical properties, layer 32 is preferably a mild or plaincarbon steel.

The second layer 36 is formed of a corrosion-resistant material, such asone of the stainless steels, i.e., noncorroding alloys of iron andchromium, including at least 12 percent chromium in order to produce therequired passivity. Layer 36 is preferably formed of a stainless steelfrom the 300 stainless series of the austenitic type, such as MS! types304 and 316.

Metallic layer 36 is not the usual stainless steel cladding bonded tothe base metal 32 before fabrication of the tank 30 by heating billetsof thematerials to be jointed to welding temperature, and then rollingthem together to produce the laminate, but is a sprayed metallic coatingof corrosion resistant material. Sprayed metal coatings are similar tothe stronger types of sintered metals and, like sintered metals, aredistinct metallurgical materials.

in metal spraying, a metal is heated to a molten or semimoltencondition, and is deposited in a finely divided form on the surface tobe coated. The molten particles flatten out upon striking the surfaceand they adhere tenaciously thereto, and to one another, producing arelatively porous structure formed of a largely plurality of castlikeparticles of the metal.

Because of the higher cost of the corrosion-resistant material comparedwith mild steel, the thickness of layer 36 should only be that requiredto perform its intended function. A layer thickness of about 0.010 to0.015 inch is suitable.

Since the stainless steels are electropositive toward a base metal orinner layer formed of a carbon steel, the pores in the stainless steelwould promote sacrificial dissolving of the base metal, which would thusdefeat the purpose of the protective coating. The inherent porosity ofthe sprayed metal coating 36, however, is used to advantage byimpregnating the pores of layer 36 with a resin having a viscosityselected to assure complete sealing of layer 36. Resin systems such aswash primers or other coatings formiilated for adhesion to stainlesssteel are suitable for this purpose.

The resin impregnated layer 36 of corrosion-resistant material has aprotective layer 38 applied to its external surfaces, which layer isformed of a more viscous organic resin system than that used toimpregnate and seal the sprayed metal coating. Coating 38, while beingillustrated as a single layer, may actually include a portion of thefirst resin used to impregnate layer 36. The impregnant used should beselected to be a primer for coating 38, which promotes adhesion betweenthe sprayed metal layer 36 and the final protective coating 38. Adhesionbetween coating 38 and sprayed metal coating 36 is promoted due to therelatively rough surface of the sprayed metal coating, providing a muchmore tenacious coating than would be obtained by applying a coating to arolled metal finish. Excellent materials for the final outer coating 38are the epoxies,-vinyls, polyurethanes, acrylics, and the like.

The disclosed construction of tank 30 has many advantages over prior artarrangements for preventing or reducing the rate of corrosion. Theoutermost protective layer 38 adheres tenaciously to sprayed metal layer36 because of the relatively rough surface of the sprayed metal coating.Scratching or imperfections in coating 38, do not reveal the base metal32, but expose a layer 36 of corrosion-resistant material, such asstainless steel. The sprayed metal layer 36 is applied as a largeplurality of minute metal particles, with each having the metallurgicalcharacteristics of cast material, not the granular-type structure whichresults from rolling. Thus, there is no possibility of intergranularcorrosion which may occur in rolled metals due to improper heattreatment. Any pores in sprayed metal layer 36 are impregnated with asealing resin system, which seals layer 36 and prevents anodic attack ofthe base metal 32. The structure of layer 36, having a large pluralityof small metal particles inhibits galvanic corrosion currents in thesprayed layer, with the insulating material in the pores aiding inbreaking up current paths. This structure reduces the incidence oflocalized attack of layer 36, called pitting corrosion, at any exposedareas of layer 36. Since layer 36 is not granular, but comprises a largeplurality of interlocked metal particles, stress-corrosion cracking isalso inhibited.

Tank 30 may be fabricated by new and improved methods which also possessadvantages over methods of the prior art. When tanks are fabricatedwholly of corrosion-resistant materials, such as stainless steel, thecost of the apparatus is not only penalized by the higher cost of thematerial, compared with the conventional mild steel, but the fabricatingcosts are also increased due to the fact that most corrosion-resistantmaterials are more difficult to form and weld. lf conventional claddingof mild steel with stainless steel is used, the initial cost of thematerial is reduced, compared with using all stainless steel, but thecladding is present during the fabrication of the tank, and care must betaken to prevent delamination of the laminated materials. Further, thecladding not only complicates the welding of the material, but specialwelding procedures and materials may be necessary in order to preventthe weld joints from being preferentially attacked by galvaniccorrosion.

The disclosed construction of the tank or casing 30 enables the tank tobe completely fabricated of mild steel, using techniques used forapparatus which is to be mounted above grade, then, after the casing ortank is completely fabricated, the sprayed metal coating may be appliedto all of the exposed surfaces, including the weld joints, whichprovides complete protection for the base metal.

FIG. 3 is a block diagram which illustrates the steps of a new andimproved method of forming corrosion-resistant electrical apparatussuitable for below grade mounting. The first steps of the method,illustrated by block 50, comprises the steps of completely fabricatingthe tank for the electrical apparatus, such as tank 16 and cover 18 oftransformer shown in H6. 1, including the welding of any brackets, andthe like, thereto. The next steps, illustrated by block 52, comprisesthe steps of cleaning and roughening the tank surface. Both of thesesteps may be performed by grit blasting the external surfaces, removingenough metal to assure that oil and other contaminants are removed fromthe tank surface, and at the same time roughening the surface of thetank to provide a receptive base for the sprayed metal coating. Thecleaning step may also be performed individually, by using suitablesolvents prior to the roughening step. The roughening step may stillinclude the grit blasting, to obtain the desired surface finish.

The next step of the method, illustrated by block 54, is to spray theexternal surfaces of the tank with corrosion-resistant material, such asone of the stainless steels. The spraying step may be accomplished witha wire gun or a powder gun. With a wire gun, the wire is melted by aflame, and is atomized by a gas which carries the metallic particlesagainst the surface being coated. If the fuel for the flame is oxygenand acetylene, and the gas used to carry the atomized particles to thesurface being coated is compressed air, extreme care must be taken toreduce the amount of oxides produced on each of the metallic particles.Stainless steel is more electronegative than its oxide,

The next step, illustrated by block 60, is to place the electricalelement in the tank 30, such as the core-winding assembly of anelectrical transformer, as well as any other auxiliary elements whichare required to complete the function of the apand each of the oxidecoated particles may, under certain conditions, act as a plurality ofgalvanic cells, which contributes to corrosion. Therefore, it ispreferable to spray the metal in a neutral atmosphere, such as nitrogen,or at least an atmosphere which is free from oxygen.

When using a powder gun, less carrier air is required to spray themetallic powder, than when spraying with a wire gun. Certain types ofpowder guns do not require compressed air, which may make it easier todevelop coatings which are oxide free. Plasma spraying, in which astream of ionized gas is produced by passing a gas through an electricarc, may also be used. While plasma spraying is more costly, it has theadvantage that oxygen and combustion gases, as well as their byproductsare not present in the gas stream, producing oxide free sprayed metalcoatings.

The next step of the method, illustrated by block 56 in FIG. 3, is toapply a first protective coating to the tank 30, over the sprayed metalcoating 36, which coating has a viscosity selected to impregnate anypores in the sprayed metal coating. The first coating of resinousinsulating material should also be selected to prime layer 36 andpromote adhesion for the final or outer layer of protective material.

The next step, illustrated by block 58 is to coat the sprayed metalcoating with a good tough moisture and chemical-resistant material, suchas the vinyls or polyurethanes with the outer protective coating 38being applied without the necessity of roughening the surface of sprayedmetal coating 36, as a sprayed metal coating is inherently rough,producing a surface which will tenaciously bond to the insulating outercoating 38.

paratus, and then, as illustrated in block 62, the insulating andcooling fluid is disposed in the tank to a predetermined level, with theinsulating fluid being a mineral oil, a synthetic liquid, or aninsulating gas. The tank 30 may then be hermetically sealed to preventthe entrance of moisture into the tank.

lnsummary, there has been disclosed new and improved electricalapparatus suitable for operation in the corrosive environmentsassociated with below grade mounting, which provides an effectivecorrosion protective system without adding significantly to themanufacturing cost of the apparatus. The tank of the apparatus may becompletely fabricated using the materials and manufacturing steps usedfor electrical apparatus manufactured for above grade use. The corrosionprotective layer is not applied to the tank until the tank has beencompletely fabricated.

The corrosion protective metallic coating, being a sprayed metalcoating, possesses unique metallurgical characteristics which, whenimpregnated with an insulating resin, aids in inhibiting localizedattack or pitting corrosion, as well as stresscorrosion cracking.lntergranular corrosion is eliminated since the sprayed structure doesnot have grains. The outer surface of the sprayed metal coating iscoated with a tough, moisture and chemical-resistant coating whichadheres unusually well to layer 38, because of its inherently roughsurface. Thus, coating 38 has better than average ability in resistingscratches and damage due to abrasion. Scratches in coating 38 onlyreveal the corrosion resisting layer 36, which is sealed by theimpregnating resin to prevent anodic attack of the base metal 32. Thesprayed metal layer 36 exhibits superior corrosion resistance, comparedwith a rolled layer of the same material, due to the unique structure ofa sprayed metallic coating.

We claim as our invention:

1. Electrical apparatus comprising:

a metallic casing,

an electrically conductive element disposed in said casing,

said electrically conductive element being adapted for connection to anelectrical potential,

and fluid means disposed in said casing,

the external surfaces of said metallic casing having a sprayed metalcoating disposed thereon, with the metal being selected for its abilityto resist corrosion,

and a protective coating disposed on said sprayed metal coating whichimpregnates the pores of the sprayed metal coating and provides aprotective outer coating on said casing.

2. The electrical apparatus of claim 1 wherein the sprayed metal coatingis a steel alloy containing at least 12 percent chromium.

3. The electrical apparatus of claim 1 wherein the metallic casing isformed of carbon steel and the sprayed metal coating is formed ofstainless steel.

4. The electrical apparatus of claim 1 wherein the protective coatingincludes first and second coats, with the first coat providing thefunction of sealing the pores of the sprayed metal coating and promotingadhesion of the second coat, said second coat including an organicresin.

5. The electrical apparatus of claim 1 wherein the metallic casing isformed of carbon steel, the sprayed metal coating is formed of stainlesssteel, and the protective coating includes an organic resin.

6. The electrical apparatus of claim 1 wherein the sprayed metal coatingis about 0.010 to 0.015 inch thick.

7. The electrical apparatus of claim 1 wherein the electricallyconductive element includes electrical transformer windings.

1. Electrical apparatus comprising: a metallic casing, an electricallyconductive element disposed in said casing, said electrically conductiveelement being adapted for connection to an electrical potential, andfluid means disposed in said casing, the external surfaces of saidmetallic casing having a sprayed metal coating disposed thereon, withthe metal being selected for its ability to resist corrosion, and aprotective coating disposed on said sprayed metal coating whichimpregnates the pores of the sprayed metal coating and provides aprotective outer coating on said casing.
 2. The electrical apparatus ofclaim 1 wherein the sprayed metal coating is a steel alloy containing atleast 12 percent chromium.
 3. The electrical apparatus of claim 1wherein the metallic casing is formed of carbon steel and the sprayedmetal coating is formed of stainless steel.
 4. The electrical apparatusof claim 1 wherein the protective coating includes first and secondcoats, with the first coat providing the function of sealing the poresof the sprayed metal coating and promoting adhesion of the second coat,said second coat including an organic resin.
 5. The electrical apparatusof claim 1 wherein the metallic casing is formed of carbon steel, thesprayed metal coating is formed of stainless steel, and the protectivecoating includes an organic resin.
 6. The electrical apparatus of claim1 wherein the sprayed metal coating is about 0.010 to 0.015 inch thick.7. The electrical apparatus of claim 1 wherein the electricallyconductive element includes electrical transformer windings.